[wasm] Stage --wasm-shared-code behind the --future flag.

R=titzer@chromium.org
BUG=v8:7424

Change-Id: Ic43f81b51c651fa826ec4d13a7eb508a27489f97
Reviewed-on: https://chromium-review.googlesource.com/1160305Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55153}

src/flag-definitions.h

@@ -622,6 +622,7 @@ DEFINE_BOOL(wasm_shared_engine, true,
 DEFINE_IMPLICATION(future, wasm_shared_engine)
 DEFINE_BOOL(wasm_shared_code, false,
             "shares code underlying a wasm module when it is transferred")
+DEFINE_IMPLICATION(future, wasm_shared_code)
 DEFINE_BOOL(wasm_trap_handler, true,
             "use signal handlers to catch out of bounds memory access in wasm"
             " (currently Linux x86_64 only)")